1. Technical Field
The present invention relates to a gas sensor that uses an ionic conductor having a characteristic electrode configuration, and a method of fabricating the same.
2. Background Art
Solid ionic conductors can be divided into two main types: oxygen ionic conductors and proton conductors. Oxygen ionic conductors are currently coming into practical use and are used in fields such as oxygen sensors and oxygen separation technology.
On the other hand, it was discovered in 1979 that a sintered body of an oxide in which SrCeO.sub.3 acts as a matrix exhibits a high level of proton conductance in an atmosphere containing hydrogen of water vapor (refer to preliminary paper 127 of the 20th Battery Forum (Nagoya), by Hiroyasu Iwahara, et al., which prompted research in the proton conductor field. However, the development of proton conductors has not yet reached a level at which they can be used in hydrogen sensors or in a hydrogen separation technology.
It is known to use a zirconia oxygen ionic conductor as an oxygen sensor. In such an oxygen sensor, platinum or a material in which platinum is the main component, such as Pt-Rh or Pt-Pd, is generally used as the electrode material. A platinum electrode is highly capable of converting oxygen gas into oxygen ions at its boundary with the zirconia oxygen ionic conductor, and moreover it is heat-resistant and extremely stable with respect to changes in the temperature of the atmosphere being measured. This platinum electrode is usually formed on the zirconia oxygen ionic conductor by a method such as chemical plating, printing, or physical vapor deposition (electron beam deposition or sputtering).
The resistance of a zirconia oxygen sensor is thought to be mainly the sum of the resistances of the zirconia ionic conductor itself and the electrode boundaries. Therefore, in order to reduce the resistance of the entire sensor, it is important to plan to reduce both the resistance of the zirconia ionic conductor itself and that of the electrode boundaries.
The boundary resistance is particularly difficult to control because it is affected by various factors such as the substance of each layer and the junction state, and the effective decrease the boundary resistance is expected. This boundary resistance is thought to be largely dependent on the state of the junction between the ionic conductor and the electrode.
One prior art method of reducing the boundary resistance between a zirconia ionic conductor layer and a platinum electrode is to increase the surface area of the zirconia ionic conductor layer, and thus increase the strength of the junction between the zirconia ionic conductor and the platinum electrode, by etching the surface with a strong alkaline liquid. However, this method does not provide sufficient adhesion between the two layers.